Letters in 2013

The insulator-to-metal transition occurring in magnetite is known as the Verwey transition, and its precise mechanism has recently come under renewed attention. Using pump–probe X-ray diffraction and optical reflectivity techniques, the dynamics of excitations known as trimerons are now examined, revealing the switching limits of this ubiquitous oxide material.

As indicated by direct band-structure measurements and calculations, tiny native imperfections in bilayer graphene are sufficient to cause the generation of coexisting massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields and has a closed-arc topology consisting of a unique chiral pseudospin texture.

Flexible devices mimicking the sensitivity of human skin typically turn pressure stimuli into electronic signals, which must be further processed to be interpreted by the user. By integrating an active matrix of organic light-emitting diodes in these foldable sensors, pressure can now control the brightness of each coloured pixel, enabling the direct visualization and quantification of the applied stimulus.

The epitaxial growth of large-area single-domain graphene on hexagonal boron nitride by plasma-assisted deposition is now reported. New sets of Dirac points are produced as a result of a trigonal superlattice potential, while Dirac fermion physics near the original Dirac point remain unperturbed. This growth approach could enable band engineering in graphene through epitaxy on different substrates.

Understanding the distribution of internal local strains within zeolites is important for catalytic applications because they can affect the rates of adsorption and diffusion of guest molecules. A ‘triangular’ deformation-field distribution in ZSM-5 zeolites is now observed, showing the presence of a strain within the crystal that arises from the heterogeneous core–shell structure.

A convincing explanation of why mixed phases of anatase and rutile TiO₂ outperform individual polymorphs is lacking. An energetic band alignment of ~0.4 eV is now shown to exist between the two phases with anatase possessing the higher electron affinity. This observation explains the separation of photoexcited charge carriers between phases and could lead to improved photocatalysts.

Making colloidal nanoparticles with controlled composition and shape is challenging because at the nanoscale surface energy favours highly symmetric structures. Now, a fast, wafer-scale fabrication scheme that combines low-temperature shadow deposition with nanoscale patterning has been developed that produces anisotropic hybrid nanocolloids with designed composition and feature sizes down to 20 nm.

Quantum wells based on mercury telluride are an experimental realization of a two-dimensional topological insulator. By using a scanning superconducting quantum interference device (SQUID) technique, the magnetic fields flowing through HgTe/CdTe heterostructures are imaged both in the quantum spin Hall and the trivial regimes, revealing the edge states associated with the quantum spin Hall state.

Controlling the direction of propagation of domain walls in magnetic nanowires is essential for their use in proposed device applications. It is now shown that Dzyaloshinskii–Moriya interactions determine the chirality of domain walls in metallic ferromagnets placed between a heavy metal and an oxide, which in turn means the direction of propagation can be determined by choosing suitable material properties.

Although the coarsening of catalytically active metal clusters can be accelerated by the presence of gases, the role played by gas molecules is difficult to ascertain. Carbon monoxide-induced coalescence of Pd adatoms supported on a Fe₃O₄ surface is now investigated at room temperature, and Pd-carbonyl species are shown to be responsible for their mobility.

At present, there are no known examples of binary icosahedral quasicrystals featuring localized magnetic moments. Now, a family of magnetic binary icosahedral quasicrystals is discovered, offering the possibility of studying the behaviour of coupled magnetic interactions in the presence of aperiodic structural order.

Results suggesting the onset of magnetism at the interface between LaAlO₃ and SrTiO₃ have been among the more intriguing associated with this system. Using element-specific techniques such as X-ray magnetic circular dichroism, direct signatures of in-plane ferromagnetic order occurring at the interface are now reported.

Difficulties in controlling the nucleation and growth of thin films of organic semiconductors have impaired progress in organic electronics. Now, efficient control of the crystallite nucleation and microstructure of a broad range of organic semiconductors without detriment to their electronic properties has been achieved through the addition of small quantities of additives—a widely used strategy in bulk polymer crystallization.

Surface-active macromolecules that are chemically different can be mixed at fluid interfaces if the molecules attract each other, or if they have complementary shapes and a net attraction is induced by a depletant. Now, a strategy that eludes the need for complementarity between the molecules, where tethered molecular brushes induce an entropic net repulsion between like species, achieves long-range arrays of perfectly mixed macromolecules.

Iridate materials are at present the focus of interest because the combination of strong spin–orbit effects and many-body electronic correlations makes their physics non-trivial. Now, the density of states of Sr₃Ir₂O₇ is mapped out spatially using scanning tunnelling microscopy and spectroscopy, yielding insights into the influence of nanoscale heterogeneities on the electronic structure.

The unconventional superconductivity associated with iron pnictide materials has been the subject of intense interest. Using an annealing procedure to control the charge-carrier concentration, the behaviour of an FeSe monolayer deposited on SrTiO₃ is now investigated, and indications of superconductivity at temperatures up to 65 K observed.

Progress in DNA-mediated nanoparticle self-assembly has been hampered by the lack of a general method to control the bonding of nanoparticles of different chemical composition into lattices by means of DNA linkers. An approach that makes possible the functionalization of any nanoparticle that has hydrophobic capping ligands with a dense monolayer of DNA, and allows for independent control of composition, particle size and lattice parameters for a variety of lattices, is now demonstrated.

A ferroelectric tunnelling heterostructure is presented in which both the height and the width of the tunnelling barrier can be electrically modulated, leading to a greatly enhanced tunnelling electroresistance. In Pt/BaTiO₃/Nb:SrTiO₃ heterostructures, an ON/OFF conductance ratio that is about an order of magnitude greater than those reported in normal ferroelectric tunnelling junctions, is demonstrated at room temperature.

Coherent twin boundaries, which usually form during the growth, deformation or annealing of crystalline solids, are widely described as perfect interfaces. Experiments and simulations now show that as-grown coherent twin boundaries in nanotwinned copper consist of incoherent segments and partial dislocations, and significantly affect the material’s mechanical behaviour and deformation mechanisms.

A highly selective and efficient approach to covalently bond complementary DNA strands in solution and on surfaces on demand is shown. The approach involves the substitution of a pair of complementary bases by cinnamate-based crosslinks, which can be activated on exposure to ultraviolet light, and allows chemical patterning of flat and curved surfaces down to micrometre and potentially submicrometre resolutions.